1. Field of the Invention
The invention relates to a polarizing plate to be used for a liquid crystal display, and more particularly to a polarizing plate having glare shield effect and electrical conductivity.
2. Description of the Related Art
In a conventional twist nematic (TN) liquid crystal display, an electric field is applied perpendicularly to a glass substrate to thereby cause liquid crystal molecules to stand perpendicularly to a panel, and images are displayed utilizing variation in light transmittance of the panel which occurs when the liquid crystal molecules are caused to stand perpendicularly to the panel. A polarizing plate generally used in such a conventional TN liquid crystal display is constituted of a pair of acrylic films, and a polarizing film sandwiched between the acrylic films. One of the acrylic films is adhered to a panel of a polarizing plate through acrylic adhesive, and a glare shield layer is formed on the other of the acrylic films adhered to a display surface. The glare shield layer is designed to have irregularities at a surface thereof, or be composed of acrylic resin in which SiO.sub.2 particles are scattered, and the SiO.sub.2 particles are fixated by being optically or thermally cured. The glare shield layer is formed for the purpose of scattering reflection lights to thereby reduce glare caused by reflection of external lights. The glare shield layer enhances visibility in display in a TN liquid crystal display.
TN liquid crystal display is employed in variety of fields because it consumes less power, is more compact, and provides better quality than other displays such as cathode ray tube (CRT). However, since crystal liquid molecules are driven perpendicularly to a panel, a viewer sees the crystal liquid molecules in different directions in dependence on an angle with which the viewer observes the panel. Since an angle of visibility, defined as an angle at which contrast is inverted, in crystal liquid molecules is quite narrow because of anisotropy in a refractive index thereof, TN liquid crystal display is not suitable to a big screen at which many viewers looks in all directions.
In these days, with development in technology for fabricating a thin film transistor (TFT) in a quite small size, a horizontal electric field type liquid crystal display which has an angle of visibility sufficiently wide to use for a big screen such as a monitor, has been put to practical use. A horizontal electric field type liquid crystal display displays images thereon, utilizing variation in light transmittance of a panel which takes place when a voltage is applied in parallel with a panel to thereby rotate liquid crystal molecules in the panel. Hence, a viewer can look at crystal liquid molecules in a common direction regardless of an angle with which the viewer looks at a panel. Thus, a horizontal electric field type liquid crystal display has almost no dependence on an angle of visibility, and has an angle of visibility as wide as an angle of visibility of CRT. A horizontal electric field type liquid crystal display is expected to take over CRT.
A polarizing plate used in a horizontal electric field type liquid crystal display has a glare shield layer on a surface thereof for enhancing visibility similarly to TN liquid crystal display. However, a horizontal electric field type liquid crystal display is accompanied with a problem that when electric charges such as static electricity are accumulated on a surface of the liquid crystal display, and the electric charges have no space to escape to, an electric field is generated perpendicularly to a panel. The thus generated perpendicular electric field puts out of order an electric field applied to liquid crystal molecules both when crystal liquid molecules are driven or not driven. As a result, crystal liquid molecules behave independently of signals transmitted thereto, and accordingly the quality of display is considerably deteriorated.
As a solution to the above-mentioned problem, a horizontal electric field type liquid crystal display has been designed to have an electrically conductive polarizing plate not only for preventing from being charged, but also for providing glare shield effect.
FIG. 1 illustrates a conventional, electrically conductive polarizing plate not only for preventing from being charged, but also for providing glare shield effect. The illustrated polarizing panel is used in a liquid crystal panel. The liquid crystal panel includes an upper glass substrate 38, a lower glass substrate 40, and a liquid crystal layer 39 sandwiched between the upper and lower glass substrates 38 and 40. A first polarizing plate includes a pair of transparent hard coat layers 43 and 44, a polarizing layer 41 sandwiched between the transparent hard coat layers 43 and 44, a glare shield layer 49 formed on the transparent hard coat layer 43, and a thin, electrically conductive film 50 formed on the glare shield layer 49. The first polarizing plate is adhered to the upper glass substrate 38 through an acrylic adhesive layer 47. A second polarizing plate includes a pair of transparent hard coat layers 45 and 46, and a polarizing layer 42 sandwiched between the transparent hard coat layers 45 and 46, and is adhered to the lower glass substrate 40 through an acrylic adhesive layer 48.
Since no electric charges are accumulated on the first polarizing plate having electrical conductivity, no electric field is generated perpendicularly to a panel, and thus it is possible to prevent deterioration of the quality of display. A polarizing plate having both an electrically conductive film and a glare shield layer, as mentioned above, has been suggested in Japanese Unexamined Patent Publications Nos. 6-324214, 6-139822, 6-196110, 61-168899, and 4-342202. However, the polarizing plates having been suggested in those Publications are accompanied with a problem which occurs when an electrically conductive film is formed.
Japanese Unexamined Patent Publications Nos. 6-324214 and 61-168899 have suggested methods of forming an electrically conductive film by evaporating electrically conductive material in vacuum. However, the suggested methods have to include an extra step of vacuum evaporation, resulting in an increase in cost for fabricating a polarizing plate. In addition, these methods use an electrically conductive, thin film. Hence, a light interference of external lights reflected from the thin film varies in dependence on a thickness of the thin film with the result that the reflected lights are colored, which considerably deteriorate quality of display.
Japanese Unexamined Patent Publications Nos. 6-139822 and 6-196110 have suggested methods of fixating electrically conductive material by burning in order to have an electrically conductive film. However, it is necessary in both the methods to burn an electrically conductive film at 100.degree. C. or greater in order to fixate the film. Namely, a polarizing plate has to be treated at a temperature greater than a maximum temperature at which the polarizing plate can withstand. Hence, these methods are not suitable for forming an electrically conductive film to be used for a polarizing plate.
Japanese Unexamined Patent Publication No. 4-342202 has suggested a method of forming irregularities on a surface of a polarizing plate for reducing reflection at the surface. The irregularities are formed by applying energy such as ultra-violet ray and heat to a base resin, scattering resin, which is cured in a different atmosphere, in a surface of a polarizing plate and then curing the resin to thereby use irregularities of the cured resin, and so on. However, this method has to have an extra step of forming irregularities, resulting in an increase in cost for fabricating a polarizing plate.
In brief, the above-mentioned prior art has the following problems.
The first problem is that external lights reflected from a surface of an electrically conductive film cause light interference in dependence on a thickness of the electrically conductive film, resulting in that the reflected lights are colored. This is because the electrically conductive film is formed as a thin film by vacuum vaporation and so on.
The second problem is that it is necessary in the prior art to carry out an extra step for forming an electrically conductive film with the result of an increase in fabrication cost of a polarizing plate. This is because a step of forming an electrically conductive film is a step additional to usual fabrication steps of fabricating a polarizing plate.